Alleviated photoinhibition on nitrification in the Indian Sector of the Southern Ocean

Nitrification, a central process in the marine nitrogen cycle, produces regenerated nitrate in the euphotic zone and emits N₂O, a potent greenhouse gas as a by-product. The regulatory mechanisms of nitrification in the Southern Ocean, which is a critical region for CO₂ sequestration and radiative benefits, remain poorly understood. Here, we investigated the in situ and dark nitrification rates in the upper 500 m and conducted substrate kinetics experiments across the Indian Sector in the Cosmonaut and Cooperation seas in the late austral summer. Our findings indicate that light inhibition of nitrification decreases exponentially with depth, exhibiting a light threshold of 0.53% photosynthetically active radiation. A positive relationship between dark nitrification and apparent oxygen utilization suggests a dependence on substrate availability from primary production. Importantly, an increased ${\mathrm{NH}}_4^+ $ supply can act as a buffer against photo-inhibitory damage. Globally, substrate affinity (α) increases with depth and transitions from light to dark, decreases with increasing ambient ${\mathrm{NH}}_4^+ $, and exhibits a latitudinal distribution, reflecting substrate utilization strategies. We also reveal that upwelling in Circumpolar Deep Water (CDW) stimulates nitrification through the introduction of potentially higher iron and deep diverse nitrifying microorganisms with higher α. We conclude that although light is the primary limiting factor for nitrification in summer, coupling between substrate availability and CDW upwelling can overcome this limitation, thereby alleviating photoinhibition by up to 45% ± 5.3%.